1. Field of The Invention
The present invention relates to a method and apparatus for determining the production capabilities of earth formations by placing a wireline formation testing tool into fluid communication with a selected formation interval and, when possible, collecting a representative sample of the connate fluids in that formation. During the sampling operation, it is also customary to obtain one or more measurements which are at least indicative of the formation pressures in the interval being sampled.
2. The Prior Art
Those skilled in the art will, of course, appreciate that many different arrangements of formation testers have been employed through the past several years. In general, these tools include either a fluid entry port or a tubular probe cooperatively arranged within a wall-engaging packer for isolating the port or probe from the well bore fluids during the test. To collect fluid samples, these prior-art tools have one or more sample chambers which are coupled to the fluid entry by a flow line having one or more control valves arranged therein. As shown in U.S. Pat. No. 3,011,554, for example,, a suitable pressure transducer is usually arranged in the flow line for transmitting pressure measurements to the surface by way of the cable supporting the tool.
Generally, the success of these tests has depended to some extent upon knowing in advance the general character of the particular formations which were to be tested, so that the sampler/tester could be equipped as required to test a formation of a given nature.
For example, where the formations to be tested were considered to be fairly competent and, therefore, not easily eroded, prior-art testers such as that shown in U.S. Pat. No. 3,011,554 have been relatively satisfactory. On the other hand, in those situations where tests were to be conducted in fairly incompetent or unconsolidated formations, it has heretofore been the practice to use testers such as those shown in U.S. Pat. Nos. 3,352,361; 3,530,933; 3,565,169; or 3,653,436. As fully described in these last-mentioned patents, each of those prior-art testing tools employs a tubular sampling member which is cooperatively associated with a conventional filter having fluid openings of a selected and uniform size for preventing the unwanted entrance of unconsolidated formation materials of a specified minimum size into the testing tool. Thus, except for dual-purpose tools such as that shown in U.S. Pat. No. 3,261,402, these prior-art formation-testing tools have been relatively successful in making tests in formations which are known in advance either to be fairly competent or to be relatively unconsolidated Moreover, since all of these prior-art testers can be operated only once during a single trip into a well bore, it has been customary to simply select in advance the particular size or type of filter believed to be best suited for a specific testing operation.
In unconsolidated formations, these tests are often impaired due to the migration of formation particles which plug the flow lines of the tool.
It has been proposed, by U.S. Pat. No. 3,926,463 a formation tester having fluid-admitting means adapted for selective movement into sealing engagement with a potentially-producible earth formation to isolate a portion thereof from the borehole fluids. The fluid-admitting means are provided with filter means having first and second cooperatively-associated filter members adapted for movement relative to one another and respectively provided with one or more filter passages that are each sized to easily pass large plugging material such as mudcake particles.
Of more significance to the previously-discussed problem of obtaining advance predictions about the potential productivity of a given formation, the known formation-testing tools, such as described in U.S. Pat. No. 3,811,321 and 3,813,936, include an enclosed test chamber which is selectively expanded for initially reducing the pressure therein and then suddenly communicated with a formation to be tested for rapidly enduing a small sample of producible connate fluids into the expanded chamber. This unique operation attempts to momentarily reduce the pressure of the incoming fluid sample for obtaining in advance a meaningful pressure-buildup curve which is at least representative of the potential productivity of the formation under test.
Similarly, U.S. Pat. No. 3,782,191 shows a formation-testing apparatus comprising filter means including selectively-operable valve means, cooperatively arranged for selective movement between an open position which allows communication between an isolated earth formation and the filter means, and a normally closed position preventing communication with the filter means.
Another known tool has been proposed, such as described in U.S. Pat. No. 3,934,468, and including a wall-engaging sealing pad carrying an extendable filter probe coaxially supported therein by a tubular valve member. Dependent upon whether the formation being tested is incompetent or competent, the filter either is advanced into the formation or remains in its normally retracted position.
Furthermore, U.S. Pat. No. 3,952,588 depicts a formation-testing apparatus having filter means including particle-collecting means cooperatively arranged for selective movement between a normally extended position for collecting plugging materials and blocking the filter means, and a retracted position for withdrawing those materials from the flow path through the now-exposed filtered fluid inlet.
Nevertheless, the known apparatus as described in the five last-mentioned U.S. patents rely on relatively complicated moving mechanism.
Typically, the prior-art filter members have been an elongated tubular member having only a plurality of narrow slits of a uniform width which are disposed either longitudinally or circumferentially. U.S. Pat. No. 3,352,361 is an example of this previous practice.
In any case, these prior-art tools have employed conventional filters having only uniformly sized filter openings which are customarily sized as dictated by the particular size of loose formation particles which were expected to be encountered during a given operation.
It has been found, however, that when these prior-art filters are used in soft formations, the pressure drop across the filtering element and the accumulated formation particles will often become so excessive that a fluid sample simply cannot be obtained in a reasonable period of time. This is easily understood, since, by virtue of the additional flow resistance imposed by the tightly-packed column of finely-divided sand particles which will be trapped in the sampling tube, most, if not all, of the flow will be through the forewardmost openings in the filter screen; thus, since, at best, little or none of the flow will be through the rearward portions of the screen, the overall flow rate will be drastically curtailed.
Moreover, experience has shown, that if the screen openings are slightly over-sized so that some sand grains will pass through the front openings, it is not at all uncommon for the sand to gradually erode the filter screen to the point that the screen is no longer effective. Thus, enlargement of the openings to improve the flow rate will often result in rapid failure of the filter.
Another known approach led to providing a screen, as shown in U.S. Pat. No. 3,864,970, in the form of a filter member designed to trap the incoming sand particles and form a compacted column of these particles and comprising slits: the rearwardmost openings are individually wider than the forewardmost openings; the several filter openings are arranged in multiple sets (of two or three slits); the respective sets have slits of incrementally increased size along almost the full length of the filter. The rear filter openings are selectively sized so that the fine particles, having entered the fluid-admitting means, bridge the rear openings, and are supposed to thereafter serve as an auxiliary filtering medium.
A more-serious problem is encountered, however, when a prior-art testing tool, equipped with a conventional filter having very narrow slits, is used to test a fairly competent or hard formation. In this situation, the usual result will be that the mudcake entering the sampling tube will swirl around inside of the tube so that the internal or inlet face of the filter screen will be quickly coated with the mudcake particles thereby plugging the narrow filter openings. Heretofore, the only practical solution to this problem has been to use a screen with the largest-possible openings that will still trap any loose formation materials which might be encountered. This obviously poses a problem where formations composed of different degrees of hardness or competency are expected to be encountered during a multi-formation testing operation such as is capable of being performed by the tool. Thus, if the filter openings are too large, sand will easily pass through the filter screen when unconsolidated formations are tested. On the other hand, if the screen openings are too small, they will be easily plugged by mudcake when hard formations are tested.
There are often situations where the performance of these known testers is significantly affected since there has heretofore been no one conventional filtering medium capable of operating efficiently with every type of earth formation. For instance, if one of these testers must be equipped with a conventional filter which is capable of stopping exceptionallyfine formation materials, the flow rate for this tester will be materially limited even when a fairly competent formation is being tested. More importantly, in situations like this, it is not at all uncommon for the necessarily-fine openings in a conventional filter to be quickly plugged by the normally-large particles of mudcake which usually line the borehole, adjacent to a potentially-producible formation. Thus, a test under these conditions will often be inconclusive, if not misleading, since it will not be known for sure whether the formation is truly unproductive or if the filter was simply plugged at the outset of the test. On the other hand where the tester is instead equipped with a conventional filter having openings designed for filtering out only fairly-large particles, there will often be an excessive induction of very-fine formation materials into the tool when the tool is testing a highly unconsolidated formation. This action will of course, frequently result in a continued erosion of the formation wall around the sealing pad so that insulated communication with the formations is quickly lost. This also causes an incomplete or inconclusive test.
It will be recognized, of course, that it is wholly impractical to change the filter in a repetitively-operable tool of this type between tests of different types of formations in a given borehole. Moreover, there is no assurance that the character of various potentially-producible formations traversed by a given borehole can even be reliably determined in advance.
Although, for the large part, the utilization of the several above-mentioned tools has been relatively successful in commercial operations, it is recognized that there is still a need for a formation testing and sampling tool with reliable filtering ability.